This invention relates to data transmission systems, and more particularly to systems for testing the integrity of data communication systems whenever particular operational faults occur.
A common class of data gathering system involves a central or "master" retrieving and control facility, communicating over fixed communication channels such as telephone lines, with spacially disparate "remote" terminals. The master has transmission and reception capability with respect to each remote, and each remote is coupled to the master by a receive wire pair and a different transmit pair. In the event that an interruption of communication occurs between the master and a remote, it is evident that the master, the remote, and the four wire transmit/receive channel are separate candidates for the cause of the fault. Of course, a malfunction at the master may be simply and conveniently dealt with; due to the disparate locations of the remote, however, adequate testing for malfunctions thereat is often inconvenient and expensive. This inconvenience and expense is most acutely felt when a person or crew is dispatched to the remote terminal, only to discover that the malfunction causing the service interruption occurred in the transmission system interconnecting the master and the remote, rather than at the remote.
It is an object of the present invention to provide apparatus, located at remote terminals, which senses communication interruptions of specified types, and which, in response thereto, automatically connects the four wire receive/transmit circuit into a "loopback" mode, thereby temporarily removing the remote terminal from the circuit and allowing testing of the transmission circuit from the master.
One constraint which is generally imposed on master-remote testing apparatus relates to the use of leased telephone circuits. That is, by far the most common transmission circuit utilized in the systems above described are telephone lines, and rigid functional constraints are imposed thereby. First, the characteristics of any apparatus hooked across a telephone line must be compatible with the telephone system voltage, current, power, and frequency characteristics and limitations. Moreover, it is understood that the proprietors of the telephone system engage in periodic tests themselves to maintain the integrity of their system.
It is a further object of the present invention to provide automatic testing apparatus which, although connected directly across a telephone line pair, not only meets functional interconnection limitations, but which furthermore is "not seen" by telephone testing apparatus and procedures, and which conducts its own sensing and testing procedures without disturbance, interruption, or error caused by spurious signals such as are involved in the tests conducted by the proprietors of the telephone circuits.
It will be appreciated that the source of communication interruption between master and remote terminals may be due to any of a variety of conditions. For example, these may include attenuation or complete loss of the transmission signal between the master and the remote (i.e., faults which would be attributed to the telephone transmission circuit), or partial or complete loss of power at the remote terminal.
It is a further object of the present invention to provide apparatus and methods for placing data systems of the type described above into a loopback mode automatically upon occurrence of complete or partial loss of power at the remote terminal, or of loss or attentuation of transmission from the master to the remote.
It is yet another object that the foregoing be accomplished while permitting flexibility and adaptability of operation, whereby fault detection and loopback is accomplished for different data characteristics, different remote terminal power supply capabilities, and different data modulation schemes.
An illustrative system to which the principles of the present invention may be applied, but without limitation thereto, is a quadra-phase modulation system. In such systems, bursts of data of predetermined duration are modulated onto a carrier. The carrier displays two characteristic frequency components. Between bursts, the unmodulated carrier is transmitted. Characteristic frequency is used herein in its standard meaning "a frequency which can be easily identified and measured in a given emission" as defined in DICTIONARY OF SCIENTIFIC AND TECHNICAL TERMS, McGraw Hill Book Co., New York, NY, 1974.